Which organism is commonly used as a bioindicator of good water quality?

• A. Diatoms
• B. Crayfish
• C. Mayfly larvae
• D. Water fleas

Answer: (C)

Assessing water quality with living indicators relies on how sensitive an organism is to pollution and habitat changes. Mayfly larvae are highly sensitive to low oxygen, turbidity, and chemical pollutants, and they require clean, well-oxygenated, cold water with stable substrates. Because of this strict tolerance, their presence in a stream signals good water quality and a healthy aquatic ecosystem. If pollution or drastic changes occur, mayfly populations decline or disappear, making their presence a reliable sign of clean conditions. While other organisms like certain diatoms can reflect nutrient levels and pollution trends, and creatures such as water fleas or crayfish may tolerate a wider range of conditions, they don’t indicate pristine water as directly. So, mayfly larvae are the organism most commonly used as a bioindicator of good water quality.